The present invention relates to a system for the measurement of error in audio-to-video timing and for the correction of such an error, and has particular, though not exclusive, relevance to television broadcasting and receiving system.
It is well known that during broadcast of television signals, the audio and video signals may be processed separately, for example, one signal may be broadcast via satellite and the other via territorial cable.
If such processing is carried out over a large enough distance, it can often be the case that the two signals arrive at their destination out of synchronisation.
There exist limits within which such temporal distortions are imperceptable to human--generally 40 ms for audio-late and 20 ms for video-late signals. Errors beyond these limits are not generally easy to quantify and correct by operator perception alone.
It is desirable that such errors be corrected so that no perceivable temporal delay occurs between the video and audio signals.
One solution is that proposed in UK Patent Application number GB 2181325A. The circuit disclosed therein performs the encoding of audio timing signals on the video signal in an encoder at the transmitter prior to transmission, and the decoding in a decoder at the receiving location of the audio timing signals from the video signal after transmission. The decoded audio timing signals are compared to timing signals derived from the transmitted audio signal, and a delay factor is generated in a unit to represent the relevant delay between the transmitted audio and video signal.
In this system, because the timing reference signal is inserted on the video signal in the vertical blanking period, processing of the video signal could involve removal and re-insertion of the blanking.